Photomultiplier tubes (PMTs) have been known as photo-sensors since previously. The photomultiplier comprises a photocathode that converts light into electrons, a focusing electrode, an electron multiplier section, and an anode, and these components are accommodated in a vacuum container. In the photomultiplier, when light is made incident on the photocathode, photoelectrons are emitted from the photocathode into the vacuum container. The photoelectrons are guided by the focusing electrode to the electron multiplier section and cascade multiplied by the electron multiplier section. The anode outputs, as signals, those electrons, among the multiplied electrons, that have reached (see for example Patent Document 1 and Patent Document 2 described below).    Patent Document 1: Japanese Patent Publication No. 3078905    Patent Document 2: Japanese Patent Application Laid-Open No. Hei-4-359855